This invention relates generally to improvements in image density detecting means. It relates more specifically to image density detecting and/or controlling mechanisms as usable in optoelectronic photo-copiers, image and character forming printers and the like image-reproducing and/or -producing machineries.
Since the present invention is most adventageously applied, to electrostatic photo-copiers, the following description will be substantially directed to the field of photo-copying technology. But, it should be noted, however, the present invention should not be limited only thereto. By adopting minor changes and modifications which may easily occur to those skilled in the art, the invention can be applied to image-forming apparatuses other than static electro-photocopiers, such as printers and the like mechanisms or machines.
As is commonly known, in the case of electrostatic photocopying machines, it is required to control the toner density so as to keep and maintain the density of copy images at a proper value at least throughout a batch of photocopying operations.
For control of the density of photo-copiers, it has already been proposed to provide on one end of the document-mounting glass plate a reference pattern of desired standard density. In order to produce toner images, having a corresponding density or tone to the reference pattern, on the photosensitive means, preferably a drum used as conventionally known, the toner image or images is/are subjected to a light projection by use of illuminous means such as LED. Then, the reflected light is sensed in terms of toner density by a density sensor (light-reception sensor) arranged in close proximity of the sensitive drum for control of toner supply quantity to a developer unit. In this case, either the regular reflection or scattered reflection is sensed and measured for determination of the image density.
It has been experienced, however, that in the case of the foregoing known process for determination of toner image density, there is a grave and inherent drawback such that when the sensor becomes fouled by the very existence of air-born toner particles floating in close proximity of and around the photosensitive drum, the toner image density as being measured must be demonstrated only in an erronous value. More specifically in this case, projection light onto the pattern plate and reflected input light from reference toner images will be considerably reduced, thereby the output from the sensor being subjected to reduction and resulting in that the correponding image density under control will be disadvantageously varied. For obviating the above drawback and according to another known proposal, correction is made against the fouling and the like troubles by use of a light quantity compensating sensor.
As an example, it is disclosed in U.S. Pat. No. 4,313,671 that the sensor output representing the reference toner image density is subjected to a correction, based upon the reflection density from the photosensitive surface of the drum before formation of toner images thereon. In this case, correction due to fouling of the above kind is made under such an assumption that the reflection density from the photosensitive drum surface without formation of tonered images is always constant.